Multi-scale approach of the interplay between mechanics and chemistry?in the regulation of the actin cytoskeleton – MuScActin

Mechanics are a constitutive part of a cell’s existence. Forces result from the cell's constant interaction with its environment, and are generated within the cell as it organizes its inner life. To achieve this, the cell relies on a complex metabolism where mechanical and biochemical aspects are intertwined, and in which the actin cytoskeleton plays a central part. To understand living cells and numerous pathologies, deciphering actin mechanics and its coupling to the activity of regulatory proteins is a key challenge today. Here, we propose to study the coupling between mechanics and biochemistry in actin dynamics using a multiscale approach, both spatial and temporal. To overcome the limitations encountered with current experimental setups, we have formed a consortium whose experimental partners have recently developed powerful new techniques to quantitatively study isolated filaments and networks. The theoretical partner develops multiscale models to describe deformation of individual filaments within networks. Our approach thus combines different spatial scales (from filaments to networks) with theoretical multiscale modeling to bridge the gap between them.
In this project, we identified three major biologically relevant questions regarding the interplay between the mechanics and the biochemistry of the actin cytoskeleton at different timescales: interaction of actin-binding proteins at short timescales, during the growth of an actin network and during its disassembly (tasks 2-4). In parallel we will inquire how single-filament deformations relate to the deformation of branched networks (task 1). This is a fundamental material science question, which will extend the reach of our multiscale experimental data.

To tackle the fundamental material science question, we will develop a new model and approach of fiber network elasticity. Our formalism will be based on the mechanical constraints induced by entanglement resulting from large nonaffine internal deformations. The large amount of statistics generated by the experiments will allow detailed comparisons with the model throughout its development, thus clarifying the ways in which entanglements develop.

To tackle the first biological question, we will study the coupling between filament and network mechanics to the binding of regulatory proteins. We will first investigate the binding activity of various actin-binding proteins (ABPs), including ADF/cofilin and different tropomyosin isoforms, as a function of applied mechanical stress. Our results will provide the first multiscale study of actin mechanical-chemical regulation, and elucidate the mechano-sensing of actin filaments.

Our next objective is to understand the construction of the network and its ability to generate forces under mechanical constraints. We will investigate the impact of single filament curvature and tension on branching. On networks, we will monitor growth velocity along with filament and branching density as a function of the applied forces. Our results should shed light on how macroscopic stresses impact the growth of actin structures.
Our last objective is to understand the reorganization of the network under mechanical constraints, over longer time scales. This is a key question to understand actin regulation in cells, where filament networks are not only assembled but also disassembled. The coupling between the activity of debranching and severing proteins involved in the ageing processes, and deformations imposed by mechanical constraints, will be studied for the first time at the scale of individual filaments and networks.
Overall, our project addresses key questions in order to elucidate the mechano-chemical regulation of actin assembly at short and long times. To achieve its ambitious goals, our project benefits from cutting-edge experimental techniques that will allow the collection of large amounts of quantitative data at different scales, in synergy with the development of theoretical models.